1. Field of the Invention
The present invention relates generally to fluid filters for filtering fluids such as air, fuel, lubricating oil and hydraulic fluid used in internal combustion engines to remove impurities such as dust and metal powders therefrom, and more particularly to an improvement in an pleated filter medium of such fluid filters thereby to attain a higher filtering ability and a longer filter life.
2. Description of the Prior Art
Most automotive internal combustion engines are equipped with air and lubricating oil filters whose filter element usually consists of a pleated filter medium formed of a sheet-type filter material such as filter paper or nonwoven fabric. The pleated filter medium is generally annular and has a plurality of longitudinally extending pleats. A pair of end plates are sealingly attached to the opposite ends of the annular filter medium by means of adhesive, securely disposing an inner perforated cylinder along the inner periphery of the annular filter medium. Additionally, a pair of fluid sealing members are securely attached to the end plates, respectively, thus constituting the filter element. With such a filter element, fluid such as air or lubricating oil to be filtered flows perpendicularly to the filter medium, i.e., the fluid is introduced through the outer peripheral face of the filter medium and gets out through the inner peripheral face of the filter medium in which dust and the like are removed and retained on the surface of the filter medium.
However, various drawbacks have been encountered in such conventional filters as discussed hereinafter. (1) Since the surface area of the inner periphery of the filter medium is considerably smaller than that of the outer periphery of the same, flow resistance of the fluid passing through the filter medium depends on the smaller surface area of the filter media inner periphery, thereby increasing the flow resistance considerably over a level corresponding to the larger surface area of the filter medium outer periphery. (2) Close contact between the adjacent pleats occurs with passage of the fluid and with progress of clogging of the filter medium. This decreases the effective filtering area of the filter medium, thereby shortening the life of the filter. (3) In a state where the filter element is installed in a casing, it is usual that the fluid to be filtered flows perpendicularly to the outer peripheral face of the filter medium and passes through the filter medium, and thereafter the flow direction of the fluid is bent approximately right angles to get out through an fluid outlet pipe provided at the central part of the casing. This increases the flow resistance of the fluid flowing through the filter. (4) The fluid introduced inside the casing through the fluid inlet pipe cannot be uniformly distributed throughout whole the outer peripheral face of the filter medium so that the fluid is sucked in larger amount through a part near the air inlet pipe and in smaller amount through a part remote from the air inlet pipe. Accordingly, the filtration rate at the part near the inlet becomes higher than an appropriate value while the fluid is difficult to flow at the part remote from the inlet. As a result, it is impossible to obtain a higher filtering accuracy and a longer life of the filter. (5) Since the pitch of the pleats is smaller at the filter medium inner periphery than at the outer periphery, the flow resistance and the life unavoidably depend on the pleat pitch of the filter medium inner periphery. Accordingly, if the number of the pleats is increased and the width of each pleat is enlarged for the purpose of lowering the flow resistance and prolong the life, effects thereon are hardly expected.